Rotary drill bits may be broadly classified into two categories: (1) drag and (2) rolling cutter. Drag bits tend to wear out quickly when used in hard rock formations. For example, our experience has been that when drilling a 61/2 inch (16.51 cm) diameter hole with a drag bit and a percussion hammer for about 100 feet (30.5 m) in very hard rock, the carbide inserts on the drag bit quickly become badly worn with the diameter of the bit being reduced to 63/8 inches (16.19 cm.). A new 61/2 inch replacement bit cannot be used since it would destroy itself in the narrower hole. A smaller diameter bit will cause deviation problems in the hole since the new bit will, most likely, not properly center itself. Moreover, it is expensive and oftentimes impossible to have on hand replacement bits having differing sizes to accomodate various drilling contingencies.
A rolling cutter bit (also called a bicone or tricone bit), originally developed for oil well drilling suffers from penetration problems after too much wear. These bits fail in the presence of very hard rock. Although their accuracy is satisfactory, oftentimes these bits fail because their small bearings cannot cope with the extraordinary high stresses experienced by the bit within the hole.
In short, current bit designs have short lifetimes; their wear patterns result in tapered holes; and worn bits cause hole inaccuracies. As a result of these difficulties, the cost per foot of drilled ground is high. The drilling industry is continuously seeking means for lowering the costs associated with drilling.